1. Field of the Invention
The present invention relates to an elevator control device for controlling raise/lower motions of a load-carrying elevator car by operating hoist ropes, each of which is connected to the car at one end and a counterweight at the other end, by driving a plurality of traction units.
2. Description of the Background Art
Conventional elevator control devices for high-speed, high-capacity elevators are designed to raise and lower an elevator car by means of a single traction unit. These conventional systems used to have such a problem that it was necessary to manufacture a high-capacity traction unit which would require a large installation space.
One previous approach directed to the resolution of this problem is found in Japanese Laid-open Patent Publication No. 2002-145544. According to the Publication, an elevator is provided with a main traction unit, auxiliary traction units and a control device which monitors operating status of the elevator. If the control device senses that a great force is needed for hoisting the elevator car, the control device actuates one or more auxiliary traction units to provide extra traction forces to aid the main traction unit.
Each of the auxiliary traction units has an interlock device for regulating transmission of a driving force from an electric motor of the main traction unit to a deflector sheave of the auxiliary traction unit by slip action to control the rotating speed and torque imparted from the electric motor to the deflector sheave.
The aforementioned system (Publication No. 2002-145544) employs the mechanical interlock device which utilizes the slip action for transmission of power to regulate the driving force transmitted from the main traction unit to the auxiliary traction units. The conventional elevator control device thus constructed has poor response characteristics and operational instability, as well as inadequate serviceability. Furthermore, there can arise relative position and speed errors among the main traction unit and the multiple auxiliary traction units due to differences in the amount of stretching of ropes caused by an imbalance of tensile forces acting on such ropes as main ropes and compensating ropes mounted on the individual traction units. This conventional mechanical system poses a problem that it is difficult to move the elevator car up and down in a stable fashion.